On Mar. 11, 2011, Japan's Fukushima nuclear accident shocked the world. One of the major reasons resulting in the nuclear accident is that the accidents caused by the earthquake and tsunami that go beyond the design basis, Mark I units produced by GE company owned by Fukushima cannot effectively remove core residual heat, resulting in core melt which produced large amounts of hydrogen and indirectly caused overpressure of the containment. Meanwhile, because of forced injection of seawater for cooling, a huge amount of radioactive substances leaked.
After the Fukushima incident, nuclear power safety agencies and the industry at large required higher capacity for long-term passive cooling of nuclear power plants. Atomic Energy Society of Japan summarizes the lessons from the accident from 12 aspects for countermeasures and suggestions. These lessons and suggestions have very significant meanings in improving the security of worldwide nuclear power plants (including passive nuclear power plant). The fourth item of them, which is lesson from the loss of all coolant, clearly proposed to consider the method of passive natural circulation cooling, which can discharge at any occasion the residual heat of the core. These valuable lessons and suggestions have important guiding significance in enhancing the existing nuclear power technology. According to lessons and suggestions from Fukushima, a nuclear power plant having an inherent safety design, with a long-term passive residual heat removal capacity can better cover the accidents beyond design basis as in Fukushima.
AP1000 is one technology of nuclear power in the world at present, its passive core cooling system is provided with a smokestack at the central top of the conical shield building, while at the top of the shield plant, an annular tank is set up around the smokestack; the central part inside the shield building at top of the steel containment is arranged with a cooling water distribution plate hanging on flying rings; spray pipes are provided at the top of the shield building; water inlet end of the spray pipes are connected to the bottom of the water tank, and the water outlet end of the spray pipes are extended above the cooling water distribution plate; an air inlet opening therethrough is provided on the outer wall of the upper end of the shield building; air deflectors are disposed between the shield building and the steel containment; the upper end of the air deflector is connected to the top of the shield building.
Although AP1000 can meet the requirements of URD, i.e., performing the maintenance of the core safety within 72 hours without depending on the operator's operation, after 72 hours, AP1000 still need an external power supply, or helicopter, fire water tankers or other equipment to help with the discharge of residual heat, so that AP 1000 is not entirely passive.